I basically agree with Aart. It happens and you don't often know why. As long as they are acting normally I don't worry about it.
Some people are more sensitive to certain foods than others. There may some of that going on with your chickens. Weather can affect them. I'm sure there are other potential causes.
My suggestion is to check them for mites and lice. You should do that occasionally anyway. You might consider getting a veterinarian to check them for worms. I wouldn't, I think I'd see other signs if they had roundworms but if it is worrying you that much it might be worth the money. But as long as they are acting normal I would not worry about it.